Although ceramic igniters have been known and commercially used for many years, the art has been plagued by in-service resistivity increases as well as premature failure of the igniters' electrical connections. Ceramic igniter production requires constructing an electrical circuit through a ceramic component, a portion of which is highly resisitive and thus rises in temperature when current is run through it from an electrical lead. However, the conductive interface between the electrical lead and the ceramic typically experiences dissimilar thermal expansion effects from the lead and the ceramic and so is susceptible to cracking. Further, undesired highly resistive zones are often created by either reaction between the metal lead and the ceramic, any other chemical interaction used in forming the combined mechanical and electrical connection, mechanical failure or chemical deterioration, i.e. oxidation. Such large increases in resistance are a problem because an igniter must be capable of igniting fuel gases throughout the lifetime of an appliance, even when voltages sink as low as 85% of the standard operating voltage (i.e., 20.4 V instead of 24.0 V) during brownouts or peak electrical demand periods. When the available voltage decreases significantly, an insufficient igniter temperature may result, particularly in older igniters in which the electrical contact has experienced severe deterioration. Hence, achieving both consistent resistivity and electrical continuity has been a continuing goal in this field.
Previous attempts at making electrical connections for ceramic igniters have had varied results. For example, U.S. Pat. No. 3,875,477 discloses a process involving (i) lightly sandblasting portions of a silicon carbide igniter in the areas where the electrical contacts are to be made, (ii) coating the sandblasted terminal ends with aluminum metal or an aluminum alloy either by dipping into molten metal or by flame spraying, and (iii) using a refractory, electrically insulating cement of the high alumina type. U.S. Pat. No. 3,928,910 discloses gas igniters having electrical leads bonded into physical slots of a ceramic (SIC) body by high temperature flame or plasma spraying which is not only intended to secure the inserted leads into their respective slots but also to fully and continuously encase the terminal parts of the igniter. U.S. Pat. No. 5,045,237 discloses molybdenum disilicide-containing ceramic igniters in which a simple machine screw and nut assembly is placed through machined holes in the ceramic body. However, the above connection means in each of these references has suffered from the problem of either substantially increased resistance with extended use, i.e., at least about 5% increase after 100,000 on/off cycles, or failing to be commercially reproducible.
The Norton Company of Worcester, Massachusetts has produced ceramic igniters in which the electrical contacts have less than about a 2% change in contact resistance after 100,000 on/off cycles. These igniters are prepared by (i) forming a ceramic igniter body having a molybdenum disilicide content of at least about 20 volume percent at the points at which the electrical contacts are to be made, (ii) painting an active metal braze on the body at those points, and (iii) soldering electrical leads to said pads by means of a solder which melts at a temperature of greater than about 500.degree. C. However, thermal expansion mismatch between the braze and the ceramic often produces cracking in the braze, leading to failure of the electrical connection.
Accordingly, it is the object of the present invention to produce a commercially viable improved ceramic igniter which
(i) will maintain a desired contact resistance after significant use, and PA1 (ii) has the desired thermal expansion characteristics in the braze. PA1 a) a lead wire, PA1 b) a ceramic substrate, and PA1 c) a braze pad having a thickness of less than about 150 microns, PA1 (a) silk screening a braze material onto the electrically conductive ceramic substrate to produce a braze pad, and PA1 (b) soldering an electrical lead to said braze pad by means of a solder which melts at a temperature of at least about 500.degree. C.